Like most "how much to charge" questions this has been covered at nauseum. Use the search feature and the advanced feature. I know I have done at least a page on how to come to a unit price on flowers, plants, you name it. Once you learn the formula you wont have to ask this question again.

“If I accept you as you are, I will make you worse; however, if I treat you as though you are what you are capable of becoming, I help you become that.”

Shade Tree,
What is your hourly rate?
What is the cost of each flat---retail or wholesale to you?
Each flat, whether they are in 6-pks or in 4" liners, each is of different cost for the flat.
We charge a set price for the flat plus labor--delivery--drivetime!
We charge the full price of a plant-tree-etc. and will plant it for half of that rate, plus delivery and cleanup.
Some people charge 35-40% markup on the total product and still get a delivery fee and cleanup charges.
This all boils down to the first question of knowing what your costs and hourly charges are!